add Lou's planter box artwork to the image tiles

Planter box SVG is here: https://github.com/streetmix/illustrations/blob/master/images/dividers/planter-box.svg
Add planter box into code

- The planter box base looks very similar to a bollard (and so I copy/pasted that to start with in variants.js).
- Fixes #478 (https://github.com/codeforamerica/streetmix/issues/478)
Merge pull request #483 from techieshark/features/add-planter-box

Add planter box